RFR: 8334015: Add Support for UUID Version 7 (UUIDv7) defined in RFC 9562 [v19]

[Jaikiran Pai]

On Thu, 9 Oct 2025 17:13:23 GMT, Kieran Farrell <kfarrell at openjdk.org> wrote:

>> With the recent approval of UUIDv7 (https://datatracker.ietf.org/doc/rfc9562/), this PR aims to add a new static method UUID.timestampUUID() which constructs and returns a UUID in support of the new time generated UUID version. 
>> 
>> The specification requires embedding the current timestamp in milliseconds into the first bits 0–47. The version number in bits 48–51, bits 52–63 are available for sub-millisecond precision or for pseudorandom data. The variant is set in bits 64–65. The remaining bits 66–127 are free to use for more pseudorandom data or to employ a counter based approach for increased time percision (https://www.rfc-editor.org/rfc/rfc9562.html#name-uuid-version-7).
>> 
>> The choice of implementation comes down to balancing the sensitivity level of being able to distingush UUIDs created below <1ms apart with performance. A test simulating a high-concurrency environment with 4 threads generating 10000 UUIDv7 values in parallel to measure the collision rate of each implementation (the amount of times the time based portion of the UUID was not unique and entries could not distinguished by time) yeilded the following results for each implemtation:
>> 
>> 
>> - random-byte-only - 99.8%
>> - higher-precision - 3.5%
>> - counter-based - 0%
>> 
>> 
>> Performance tests show a decrease in performance as expected with the counter based implementation due to the introduction of synchronization:
>> 
>> - random-byte-only   143.487 ± 10.932  ns/op
>> - higher-precision      149.651 ±  8.438 ns/op
>> - counter-based         245.036 ±  2.943  ns/op
>> 
>> The best balance here might be to employ a higher-precision implementation as the large increase in time sensitivity comes at a very slight performance cost.
>
> Kieran Farrell has updated the pull request incrementally with one additional commit since the last revision:
> 
>   update spec

src/java.base/share/classes/java/util/UUID.java line 74:

> 72:  *
> 73:  * @spec https://www.rfc-editor.org/rfc/rfc9562.html
> 74:  *      RFC 9562 Universally Unique IDentifiers (UUIDs)

Hello Kieran, In this class there's a pre-existing `variant()` method. On that method, the javadoc still refers to RFC-4122. Please update `<a href...` link in that javadoc to use RFC-9562 like you have done here and please also remove the `@spec` from that method's javadoc. I don't think it's needed there, given that it will link to the RFC as well as this class level `@spec`

test/jdk/java/util/UUID/UUIDTest.java line 150:

> 148:     }
> 149: 
> 150:     private static void epochMillis_userInputTest() {

Nit - it might be better to name this method `testEpochMillisTimestamp()`

test/jdk/java/util/UUID/UUIDTest.java line 159:

> 157:             }
> 158:         } catch (Exception e) {
> 159:             throw new AssertionError("Unexpected exception with timestamp " + timestamp + ": " + e);

Please pass the original exception `e` as the cause of this AssertionError, something like:

throw new AssertionError("Unexpected exception with timestamp " + timestamp, e);


That will help debug any failures if at all they happen. Same comment for a few other places in this new test method.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2452711341
PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2452716220
PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2452715029